Deep underground: The red lines show where the fortress buildings lie beneath the present-day prison structures

'But what we don't really know is
what exactly became of the fortifications, what state they are in and
what is left of the cultural resources. And although it is not always
desirable to excavate, with geophysics we can help people to know what
is below the surface without actually disrupting it.'

The
Texas A&M team is using a radar that penetrates the ground and send
signals to a receiver, which reflects the structures under the ground.

Scans
of the prison yard have revealed a subterranean tunnel system that
would have been used to transport people and ammunition, and would have
been covered in earth to protect it.

Former life: Taken in 1868, this photo shows one of the many cannons that formed the garrison

Military history: In 1903 it was turned into a military prison, then it was made into a state penitentiary in 1934

'We get signatures that indicate there is not only a tunnel, but magazine buildings too.'

Another expert, California State University Chico's Dr Tanya Wattenburg Komas, says parts of the fortifications were earthen and covered over with concrete that would have arrived in barrels from Europe, since the U.S. was not producing its own at that time.

It is some of the oldest concrete in existence in the United States.

Fort buildings: Testing on the parade ground shows that a large structure exists beneath the ground that researchers believe to be a caponier

Locked up: Prisoners would spend most of their time in tiny cells on the island, occasionally spending an hour in the yard beneath which lay the remains of the fortress

Everett and his team believe the most exciting discovery is lying in wait for them under the parade ground at the southern end of the island.

'It is called a caponier, and it is a large structure that juts out into the bay and provides defensive cover. We have seen it in the old photographs but it has completely disappeared from present view,' Everett told BBC News.

The radar shows a very large structure exists beneath the ground.

Archaeologists hope to begin excavating the area soon.

Alcatraz Federal Penitentiary was home to the 'worst of the worst.' During its 29-year life as a prison, beginning in 1934, some of the country's most notorious prisoners passed though its cells, including Al Capone and James 'Whitey' Bulger.